1. Field of the Invention
This invention relates to a four-wheel steering system for a vehicle in which both the front wheels and the rear wheels are turned in response to operation of the steering wheel.
2. Description of the Prior Art
In Japanese Unexamined Patent Publication 59(1984)-81272, there is disclosed a four-wheel steering system in which a pinion of a steering shaft for turning the front wheels is engaged with a ring gear of a differential gear, one of the side gears of the differential gear is connected to an electric motor and the other side gear is connected to a shaft for turning the rear wheels and in which the rear wheel turning characteristics, that is, which direction the rear wheels are turned in for a given turning direction of the front wheels, in the same direction (the same phase) or reverse direction (the reverse phase) and the rear wheel turning angle ratio (the ratio of the turning angle of the rear wheels to the turning angle of the front wheels), are changed by controlling the electric motor. Further, in U.S. Pat. No. 4,418,780, there is disclosed a four-wheel steering system in which the rear wheel turning angle ratio is changed according to the vehicle speed.
In the case that the rear wheel turning angle ratio is changed by controlling an electric motor, the actual rear wheel turning angle ratio can be deviated from a desired rear wheel turning angle ratio when the electric motor itself or the control system thereof temporarily malfunctions due to disturbance. This can be avoided by comparing the actual rear wheel turning angle ratio with the desired rear wheel turning angle ratio and correcting the actual rear wheel turning angle ratio based on the result of the comparison. However, when the rear wheel turning angle ratio is always feedback-controlled on the basis of the output of a rear wheel turning angle ratio sensor, it is difficult to obtain quick response due to the time required for signal processing and hunting is apt to occur, that is, the actual rear wheel turning angle ratio is apt to fluctuate about the desired rear wheel turning angle ratio.